Description
This instrument is a circular slide rule, a variation on the most common type, also shown in this drawer, and described below.

Developed during the seventeenth century, the modern slide rule is based upon the design by William Oughtred (circa 1630). It is one of many calculation devices that is based on the logarithmic scale, a calculation method invented in 1614 by John Napier.

Before the rise of the pocket electronic calculator in the 1970s, the slide rule was the most common tool for calculation used in science and engineering. It was used for multiplication and division, and in some cases also for ‘scientific’ functions like trigonometry, roots and logs, but not usually for addition and subtraction.

A logarithm transforms the operations of multiplication and division to addition and subtraction according to the rules log(xy) = log(x) + log(y) and log(x/y) = log(x) - log(y). The slide rule places movable logarithmic scales side by side so that the logarithms of two numbers can be easily added or subtracted from one another. This much simplifies the alternative process of looking up logs in a table, thus greatly simplifying otherwise challenging multiplications and divisions. To multiply, for example, you place the start of the second scale at the log of the first number you are multiplying, then find the log of the second number you are multiplying on the second scale, and see what number it is next to on the first scale.

Circular slide rules allow the longest dimension to be reduced by a factor of about 3 (i.e. π).